The present invention relates to the art of radio frequency interface circuits. The invention finds particular application in interfacing between the transmission and receipt of magnetic resonance spectroscopy probe signals and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will find application in magnetic resonance imaging, magnetic resonance chemical or physical analysis, and other applications in which the same coils function alternately as radio frequency transmission and reception antennae.
Usually, magnetic resonance spectrometers have used linear RF polarization for simplicity of design, although worthwhile gains can be had by using quadrature coils. Heretofore, magnetic resonance spectrometers using quadrature coils have commonly included a source of radio frequency driving signals that include two components which are offset by 90 degrees. A radio frequency transmitter and a signal splitter or divider are frequently employed to achieve this phase shift. The two radio frequency transmission signal components are conveyed through parallel circuits and parallel matching networks to a pair of quadrature probes. The quadrature probes are connected to parallel reception paths. The reception paths might each include a quarter wave line and diode terminations or other means to function as a double throw switch for switching between transmitting and receiving. Moreover, each parallel path may include a preamplifier, rectifier, analog-to-digital converter, and other receiver and signal processing components.
One of the disadvantages of the prior arts systems was the inability to maintain signal fidelity over a wide range of signal strengths, e.g. a 50 db RF envelope. The prior arts systems tended to distort low level portions of the envelope by causing distortion of the pulse shape, alterations in the relative amplitude, clipping of wave portions, or even loss of signal portions entirely.
Another problem resides in incomplete isolation of the transmitter and receiver. The transmitter may degrade the received signals increasing the noise level of the radio frequency signal. The increased noise may be attributable to the injection of noise into the receiver from the transmitter or the loading of the probe output by the transmitter.
Still further difficulties arise in selecting components which function satisfactorily adjacent high magnetic fields. Hybrid transformers, ferrite-core transformers, or other transformers incorporating high permeability materials become saturated by the high magnetic field associated with magnetic resonance imaging. Placing the transformers a sufficient distance from the magnetic field causes additional signal losses in the transmission lines and the possibility of phase shifts. The substitution of air core hybrid transformers complicates electronic design and tends to reduce performance.
The present invention provides a new and improved interface which eliminates the above referenced problems and others to achieve improved magnetic resonance imaging and spectroscopy.